This invention relates to a method of preparing a carbonaceous pitch useful for use as a precursor material for carbon fibers.
As carbon fibers are increasingly applied in many fields, an attempt is now made to use them for incorporation into bodies of plastics, ceramics, concretes and metals. As precursor materials for carbon fibers, polyacrylonitrile fibers have been hitherto employed. Since the carbon fibers obtained from polyacrylonitrile fibers are expensive, however, a number of studies have been made in recent years to utilize carbonaceous pitch as raw materials for carbon fibers.
The general method for the production of carbon fibers from carbonaceous pitch includes melt spinning pitch into fibers, rendering the spun fibers infusible, and carbonizing the infusible fibers. To smoothly perform such a method, the properties of the raw material pitch are very important. The most important requirement is that the pitch must have a good spinnability. It is also important that the pitch must have properties so that the spun fibers obtained therefrom may be rendered infusible and carbonized without difficulty. Pitch which can meet with the above criteria has been hitherto considered to be of a type which is obtained by carefully thermally treating a raw material oil, such as a naphtha cracking residue, a recycle oil in fluidized bed catalytic cracking processes or a coal tar, which has a high content of aromatic components and a low content of impurities such as metals, inorganic matters and sulfur components. Thus, pitch derived from a low grade oil such as a vacuum residue or an atmospheric distillation residue has been considered to be unsuitable for use as a raw material pitch for the production of carbon fibers, since such a pitch has a poor spinnability.
Since the high grade raw material oils described above are relatively expensive and fail to give pitch with a high yield, the carbon fibers obtained therefrom are also expensive. Therefore, there is a great demand for carbonaceous pitch which is inexpensive and which has properties suitable for the production of carbon fibers.
In Japanese Published Unexamined Patent Application No. 57-100186, there is disclosed a method in which a mesophase pitch is hydrogenated to render the mesophase substantially soluble in quinoline. The pitch obtained by this method is termed "dormant mesophase pitch". The dormant mesophase pitch is optically isotropic in nature and, when heated above its melting point, is a homogeneous liquid in a single phase. When subjected to shear forces in one direction, the dormant mesophase pitch is converted into the optically anisotropic state by orientation, in the direction parallel to that direction, of its latently optically anisotropic components formed during the hydrogenation treatment of the mesophase pitch. While the dormant mesophase pitch has a good spinnability, the process for the production thereof has a drawback because it requires a hydrogenation step which makes the resulting pitch expensive.